X-rays or gamma rays from a radioactive source or gamma radiation source have uses in industry and medical application. For example, X-rays or gamma rays may be used in industry for non-destructive testing of parts or components by X-ray inspection. A workpiece under inspection may be placed between an X-ray source or gamma ray source and a detector to inspect the workpiece for any abnormalities. In another non-destructive test arrangement, the detector may also be placed to receive X-rays or gamma rays that are back-scattered from the workpiece to detect any abnormalities. Similarly, X-rays and other radioactive materials are used in medical science for both diagnosis and therapeutic purposes. However, exposure to X-rays or gamma rays, or prolonged or repeated exposure to even low intensity X-rays or gamma rays may be harmful. X-rays are invisible to the human eye, so users of X-ray equipment wear sensors or monitors to indicate that X-rays are on and/or wear film badges that show X-ray exposure has taken place. A warning indicator, such as an illuminated sign or other alerting device is typically operated to warn users and others that such equipment is active or in use. Under some circumstances, however, such warning devices may not function properly or may be inoperable for whatever reason, and film badges and similar detection devices may only identify exposure after the exposure has occurred. Accordingly, there is a need to provide an arrangement or mechanism that provides an instantaneous visual indication that an X-ray source or gamma source is exposed or unshielded and that is not subject to the disadvantages of devices that may require electrical power for illumination and operation of a switch or other mechanism when the X-ray source or gamma source is active, exposed or not properly stored in a shielded container.